Method and apparatus for inspecting spark plug while spark plug is installed in engine

ABSTRACT

Method and apparatus for inspecting a spark plug as installed in an engine, for an abnormal size of a discharge gap and cracking of porcelain, based on a shape parameter value Sn (=Vn/Tn) calculated from a voltage parameter value Vn which is an average of primary voltage V 1  during a period of substantially inductive discharge, and a time parameter value Tn indicative of the period of the substantially inductive discharge. The inspection for the abnormal discharge gap size is effected with the engine held at rest, while the inspection for the porcelain cracking is effected with the engine controlled by a motor so as to increase air pressure within the cylinder bore, contrary to conventional system wherein the inspection is effected during operation of the engine by combustion of an air-fuel mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a method and an apparatusfor inspecting a spark plug or ignition plug for defects thereof such asdefects of a discharge gap and a porcelain thereof, and moreparticularly to such method and apparatus which permit the inspection ofthe spark plug while the spark plug is installed in an engine.

2. Discussion of related Art

Since spark plugs are relatively likely to be defective duringinstallation thereof in an engine, the inspection of the spark plugs isdesirably conducted after the installation in the engine. An example ofa method of inspecting spark plugs for their discharge gaps while thespark plugs are installed in an engine. According to this inspectionmethod, the secondary voltage of an ignition coil is detected by anexclusive probe while the engine is operated by combustion of anair-fuel mixture, so that the spark plug is inspected for adequacy ofthe discharge gap based on a time duration of an inductive dischargecomponent of the detected secondary voltage.

This inspection method permits the inspection of the spark plug asinstalled in the engine. However, this method is a so-called “firingcheck” in which the engine is operated by combustion of the air-fuelmixture. Further, the method relies on only the time duration of thedischarging. Therefore, the conventional inspection method cannot beconsidered satisfactory. The discharging in the spark plug tends to beinfluenced by the pressure and the fuel concentration of the air-fuelmixture surrounding the discharge gap of the spark plug. In the firingstate of the engine, there exists a relatively strong fluid flow withinthe cylinder of the engine, which may cause considerable variations inthe pressure and fuel concentration around the discharge gap, leading toinsufficient accuracy of inspection of the discharge gap.

It is desirable to inspect the spark plug for a defect of the porcelain,as well as for the adequacy of the discharge gap. The conventionalinspection method indicated above cannot satisfy this need.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide amethod of inspecting a spark plug, which is improved over theconventional method disclosed in JP-A-50-74034.

A second object of the invention is to provide an apparatus suitable forpracticing the method of the invention.

The first object indicated above may be achieved according to any one ofthe following features of the present invention, which are numbered toindicate possible combinations of the features:

(1) A method of inspecting a spark plug while the spark plug isinstalled in an engine, characterized in that a state of the spark plugis determined on the basis of at least one voltage-related quantityrelating to a voltage which is applied to the spark plug without theengine being supplied with a fuel, such that a level of the voltage ishigh enough to cause the spark plug to generate a spark.

The present method may be practiced while the pressure within a cylinderof the engine for which the spark plug is provided is maintained at theatmospheric pressure or at a level higher than the atmospheric pressure.Further, the method may be practiced while the engine is held at rest oroperated by an external drive device. The operation of the engine by theexternal drive device is performed, without a fuel being supplied to theengine, namely, without firing of the engine, so as to raise thecylinder pressure at which the engine is fired during normal operationby combustion of the fuel. The inspecting apparatus for practicing thepresent method may be adapted to utilize the ignition system of theengine per se. However, where the inspection of the spark plug iseffected by simply raising the cylinder pressure of the engine, this maybe accomplished by closing at least one of the intake and exhaust portsand introducing a compressed gas such as compressed air into thecylinder through the closed port.

In any of the above cases, the inspection is effected without the enginebeing supplied with a fuel, whereby the inspection would not beinfluenced by the fuel concentration of the air-fuel mixture. Further,since the engine is held at rest or operated by the external drivedevice without firing thereof, the amount of flow of the fluid withinthe cylinder is made smaller than where the inspection is effected withthe engine being fired, so that the pressure variation in the vicinityof the discharge gap of the spark plug is reduced, resulting in animprovement in the accuracy of inspection of the spark plug for thedischarge gap size, and leading to enhanced reliability of theinspection.

For inspecting the spark plug, it may be energized with a secondaryvoltage which is generated by a voltage applying device on the basis ofa primary voltage lower than the secondary voltage. In this case, theabove-indicated at least one voltage-related quantity may consist of atleast one of the primary and secondary voltages and a time duration of adischarge which occurs on the energized spark plug. This arrangementutilizing the two or more kinds of information permits a furtherimprovement in the inspection reliability. Even where the inspection isbased on at least one of the primary and secondary voltages, withoututilizing the discharge time duration, the number of kinds of theinformation that can be utilized for the inspection is larger than wherethe inspection is based on only the discharge time duration. Forinstance, the voltage information may include an average of the primaryor secondary voltage, a rate of change of the voltage and a waveform ofthe voltage.

(2) A method of inspecting a spark plug while the spark plug isinstalled in an engine, characterized in that the spark plug isenergized with a secondary voltage which is generated by a voltageapplying device on the basis of a primary voltage lower than saidsecondary voltage, such that a level of said secondary voltage is highenough to cause the spark plug to generate a spark, and that a state ofthe spark plug is determined on the basis of at least one of the primaryand secondary voltages.

In the method according to the feature (2) of the invention whichutilizes at least one of the primary and secondary voltages, the numberof kinds of the information on which the inspection is effected islarger than where the inspection is based on only the discharge timeduration, as described above with respect to the feature (1) of theinvention. Accordingly, the inspection reliability is improved. However,it is to be understood that the method according to the present feature(2) does not exclude the inspection while the engine is fired bycombustion of a fuel, and the inspection which utilizes the dischargetime duration as well as the at least one of the primary and secondaryvoltages.

(3) A method according to the above feature (1) or (2), wherein thespark plug is energized by application of the primary voltage orsecondary voltage thereto while the engine is operated by an externaldrive device.

(4) A method according to any one of the above features (1)-(3), whereinthe spark plug is energized by application of the primary voltage orsecondary voltage thereto while the engine is held at rest.

The method according to the features (3) and (4) may include a step ofinspecting the spark plug while the engine is held at rest, and a stepof inspecting the spark plug while the engine is operated. Since arelatively large number of kinds of information may be obtained whilethe engine is held at rest and while the engine is operated, the numberof items of the inspection may be increased, and the inspectionreliability is accordingly improved.

(5) A method according to any one of the above features (1)-(4), whereinthe spark plug is energized by application of the primary voltage orsecondary voltage thereto while a pressure in a cylinder of the enginefor which the spark plug is provided is held at an atmospheric level.

The method according to this feature (5) includes a step of inspectingthe spark plug while the engine is held at rest with the cylinderpressure maintained at the atmospheric pressure. However, the method mayinclude a step of inspecting the spark plug with the engine beingoperated by an external drive device while at least one of the intakeand exhaust valves of the engine is held open. This step may beperformed by utilizing the ignition system of the engine per se, as apart of the inspection apparatus. In this case, the ignition system maybe inspected for any abnormality, without an influence of a variation ofthe pressure within the cylinder of the engine.

(6) A method according to any one of the features (1)-(5), wherein thespark plug is energized by application of the primary voltage orsecondary voltage thereto while a pressure in a cylinder of the enginefor which the spark plug is provided is held at a level higher than anatmospheric pressure.

The method according to the features (5) and (6) may include a step ofinspecting the spark plug while the pressure in the cylinder is held atthe atmospheric level, and a step of inspecting the spark plug while thepressure is held at an elevated level higher than the atmosphericpressure. In this case, a relatively large number of kinds ofinformation may be obtained while the cylinder pressure is held at theatmospheric level and at the elevated level, so that the number of itemsof the inspection may be increased, whereby the inspection reliabilityis accordingly improved. For instance, the spark plug may be inspectedfor a defect of the discharge gap while the cylinder pressure is held atthe atmospheric level, and for a defect of the porcelain structure whilethe cylinder pressure is held at a level higher than the atmosphericlevel.

(7) A method according to the above feature (6), wherein the pressure inthe cylinder is substantially equal to a pressure in the cylinder whenan air-fuel mixture in the cylinder is ignited by combustion of a fuelduring normal operation of the engine.

(8) A method according to any one of the above features (1) and (3)-(7),wherein the spark plug is energized with a secondary voltage which isgenerated by a voltage applying device on the basis of a primary voltagelower than the primary voltage, and the at least one voltage-relatedquantity consists of at least one of the primary and secondary voltages.

(9) A method according to the above feature (2), wherein the at leastone of the primary and secondary voltages is detected while the engineis operated by combustion of a fuel.

(10) A method according to any one of the above features (1)-(9),wherein the state of the spark plug is determined also on the basis of atime duration of a discharge which occurs on the spark plug energized toproduce the spark.

(11) A method according to the feature (10), wherein the state of thespark plug is determined on the basis of a ratio of an average of theabove-indicated at least one of the primary and secondary voltages tothe time duration of the discharge.

The second object indicated above may be achieved according to anotheraspect of the invention, which provides an apparatus for inspecting aspark plug while the spark plug is installed in an engine and while theengine is not supplied with a fuel, the apparatus comprising: (a) avoltage applying device for applying to the spark plug a voltage whichis high enough to cause the spark plug to generate a spark; and (b) amonitoring device for detecting at least one voltage-related quantityrelating to the voltage, and determining a state of the spark plug onthe basis of the detected at least one voltage-related quantity.

The second object may also be achieved according to a further aspect ofthe invention, which provides an apparatus for inspecting a spark plugwhile the spark plug is installed in an engine, the apparatuscomprising: (a) a voltage applying device for applying to the spark pluga secondary voltage which is generated on the basis of a primary voltagelower than the secondary voltage such that a level of the secondaryvoltage is high enough to cause the spark plug to generate a spark; and(b) a monitoring device for detecting at least one of the primary andsecondary voltages upon application of the secondary voltage to thespark plug, and determining a state of the spark plug on the basis of atleast the at least one of the primary and secondary voltages.

BRIEF DESCRIPTION OF DRAWINGS

The above and optional objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings, in which:

FIG. 1A and 1B are views showing a spark plug without defects of adischarge gap and a porcelain thereof;

FIG. 2A and 2B are views showing a spark plug with a defect of adischarge gap thereof;

FIG. 3 is a view showing a spark plug with a defect of a porcelainthereof;

FIG. 4 is a schematic block diagram schematically showing an inspectionsystem used for practicing a spark plug according to one embodiment ofthis invention;

FIG. 5 is a graph indicating a relationship between a capacitivedischarge and an inductive discharge in a waveform of a primary voltageobtained by the inspection system of FIG. 4;

FIG. 6 is a front elevational view schematically showing an entirearrangement of the inspection system;

FIG. 7 is a graph indicating a general relationship (Paschen's law)among the size of a gap between two parallel electrodes, ambient airpressure and an electrode potential at which sparking is initiated;

FIG. 8 is a graph indicating a waveform of a primary voltage obtained bythe inspection system in a normally installed state of the spark plug;

FIG. 9 is a graph indicating a waveform of a primary voltage obtained bythe inspection system in the presence of a defect of a discharge gap ofthe spark plug;

FIG. 10 is a graph indicating a waveform of a primary voltage obtainedby the inspection system in the presence of a defect of a porcelain ofthe spark plug;

FIG. 11 is a flow chart illustrating an ignition monitoring routineexecuted by a processing unit of a monitoring device of the inspectionsystem;

FIG. 12 is a circuit diagram showing an arrangement of a monitoringdevice of the inspection system according to another embodiment of theinvention alternative to that of FIG. 11;

FIG. 13 is a graph indicating waveforms of a primary voltage obtained ina normally installed state of the spark plug, and in defective states ofthe discharge gap and the porcelain of the spark plug, respectively;

FIG. 14 is a flow chart illustrating an ignition monitoring routineexecuted in a further embodiment of the invention, in place of theroutine illustrated in the flow chart of FIG. 11; and

FIG. 15 is a flow chart illustrating a sub-routine executed in step S202of the ignition monitoring routine of FIG. 14, for calculating anaverage primary voltage meanV1S.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will be described preferred embodiments of a spark plug inspectionmethod of the present invention, together with some arrangements of aninspection system suitable for practicing the method. The inspectionsystem is capable of inspecting the spark plug for defects of thedischarge gap and the porcelain, independently of each other, during aprocess of assembling an engine.

Referring first to FIG. 1A, there is shown a spark plug 10 normallyinstalled in an engine, without any defects of a discharge gap and aporcelain thereof. The discharge gap is a gap formed or defined by andbetween a center electrode 12 and a grounding electrode 14 of the sparkplug 10. In FIG. 1B, there is also provided an enlarged view of aportion of the spark plug 10 in the vicinity of the discharge gap. Thesize of the discharge gap is indicated at “g” in FIG. 1.

On the other hand, the spark plug 10 shown in FIG. 2A is an examplehaving a defect of the discharge gap. In FIG. 2B, there is also providedan enlarged view of the portion of the spark plug 10 in the vicinity ofthe discharge gap. The size of the defective discharge gap is indicatedat “g′” in FIG. 2. The defect of the discharge gap (hereinafter refereedto as “discharge gap defect” where appropriate) may be caused by plasticdeformation of the grounding electrode 14, which may take place due to afall of the spark plug 10 or collision of the spark plug 10 with theengine upon installation of the spark plug 10 in the engine. In thespecific example of FIG. 2, the free end portion of the groundingelectrode 13 is located nearer to the center electrode 12 so that thedischarge gap g′ is smaller than the nominal value g of the normallyinstalled spark plug 10 of FIG. 10. This discharge gap defect isindicated at GD in FIG. 2. The ignition of the compressed air-fuelmixture becomes difficult as the discharge gap decreases, since aninsufficient size of the discharge gap does not permit the formation ofa nucleus of a flame which is necessary for promotion of the combustionof the air-fuel mixture.

The spark plug 10 shown in FIG. 3 is an example having a defect of aporcelain 16, as indicated at PC in the figure. The defect of theporcelain 16 (hereinafter referred to as “porcelain defect”, whereinappropriate) may be pin holes or cracks formed in the porcelain 16,which is provided for insulation between the center and groundingelectrodes 12, 14. While most of the pin holes are formed duringmanufacture of the porcelain 16, the cracking may occur not only duringmanufacture of the porcelain 16 but also due to a fall of the spark plug10, or collision of the porcelain 16 with the engine or an excessivelylarge force acting on the porcelain 16 during installation of the sparkplug 16 in the engine. If the pin holes or cracks have a relativelylarge size, the engine may suffer from an energy loss. In this respect,it is noted that a shell 18 of the spark plug 10 as installed in theengine is grounded like the grounding electrode 14, so that a dischargeor sparking may take place between the center electrode 12 and the shell18, in the presence of the pin holes or cracks in the porcelain 16, asdescribed below. This is a typical example of the porcelain defect.

Referring next to the schematic block diagram of FIG. 4, there isschematically illustrated an inspection system capable of inspecting thespark plug 10 for the discharge gap defect and the porcelain defect,independently of each other. The independent inspection of the dischargegap and porcelain defects mean that the inspection of the spark plug 10for the discharge gap can be achieved irrespective of the presence orabsence of the porcelain defect, while the inspection of the spark plug10 for the porcelain defect can be achieved irrespective of the presenceor absence of the discharge gap defect. The present inspection systemincludes an inspecting device 20 and a control device 24 as majorcomponents thereof. The inspecting device 20 includes an ignition coil30, a transistor 32, an ignition control device 36, a monitoring device38 and a motor 40.

The ignition coil 30 has a primary coil 42 and a secondary coil 44. Oneend of the coil 30 and one end of the coil 42 are connected to eachother and to a voltage source (which provides a line voltage Vss). Whileonly one set of the transistor 32, ignition coil 30 and spark plug 10 isshown in FIG. 4, an engine 48 to be inspected by the present inspectionsystem has a plurality of sets of these components 32, 30, 10 whichcorrespond to respective cylinders of the engine.

The other end of the primary coil 42 is connected to a collector C ofthe transistor 32. The voltage at the connection between the collector Cand the primary coil 42 is a primary voltage V1. The transistor 32 hasan emitter E connected to the ground, and a base B connected to theignition control device 36 and the monitoring device 38. The other endof the secondary coil 44 is connected through a high voltage wire to thecenter electrode 12 of the spark plug 10. The voltage at the connectionbetween the secondary coil 44 and the center electrode 12 is a secondaryvoltage V2. With this secondary voltage V2 applied across the electrodes12, 14, a discharge takes place at the discharge gap of the spark plug10, namely, a spark jumps across the discharge gap. It will beunderstood that the ignition coil 30, the transistor 32 and the ignitioncontrol device 36 cooperate to constitute a voltage applying device forenergizing the spark plug 10 by application of the secondary voltage V2.

The control device 24 is adapted to apply control signals CTRLC andCTRLJ to the ignition control device 36 and the monitoring device 38.Each of these control signals CTRLC, CTRLJ has two kinds. That is, thesignal CTRLC has a first and a second control signal CTRLC1 and CTRLC2,while the signal CTRLJ has a first and a second control signal CTRLJ1and CTRLJ2. The first control signals CTRLC1, CTRLJ1 are applied to thedevices 36, 38, respectively, to inspect the spark plug 10 for thedischarge gap defect, while the second control signals CTRLC2 and CTRLJ2are applied to the devices 36, 38, respectively, to inspect the sparkplug 10 for the porcelain defect. The application of the first controlsignals CTRLC1, CTRLJ1 will be first explained. The control device 24 isadapted to apply the first control signal CTRLC1 to the ignition controldevice 36 for inspecting the spark plug 10 for the discharge gap defect.

In the engine 48 to be inspected by the present inspection system, thenumbers of the transistors 32 and the ignition coils 30 are the same asthe number of the ignition plugs 10. However, some engine has only onetransistor 32 and only one ignition coil 30. For instance, an engine mayhave a distributor for connecting the secondary coil 44 of the same oneignition coil 30 selectively to the center electrodes 12 of theplurality of spark plugs 10. In this type of engine, the centerelectrodes 12 of the spark plugs 10 may be selectively connected to thesecondary coil 44 of the ignition coil 30 through high-voltage ignitionpoint relays or solid state relays corresponding to the spark plugs 10,so that a selected one of the spark plugs 10 which is to be inspectedcan be energized to cause a spark by electrically controlling the statesof the ignition point relays or solid state relays.

Upon reception of the first control signal CTRLC1 from the controldevice 24, the ignition control device 36 immediately applies arectangular pulse MPLS to the base B of the transistor 32 and themonitoring device 38. At this time, the engine 48 is controlled suchthat at least one of the intake and exhaust valves of the cylinderprovided with the spark plug 10 corresponding to the transistor 32 towhich the rectangular pulse MPLS is applied is placed in an openposition in which the pressure around the discharge gap is made equal tothe atmospheric pressure. This control of the engine 48 is accomplishedby operation of the motor 40. The motor 40 is preferably equipped with aservo mechanism that permits the angular position of the crankshaft ofthe engine 48 to be changed as needed. The rectangular pulse MPLS isapplied to only a selected one of the transistors 32 corresponding tothe engine cylinders. This selected transistor 32 corresponds to thespark plug 10 to be inspected, which is shown in FIG. 4 by way ofexample. In other words, the spark plug 10 corresponding to thetransistor 32 to which the rectangular pulse MPLS is applied isinspected for the discharge defect.

For inspecting all of the spark plugs 10, the rectangular pulse MPLSmust be applied to the corresponding transistors 32. On the other hand,the monitoring device 38 receives the rectangular pulse MPLS when thispulse is applied to each of the transistors 32. The function of themonitoring device 38 will be described. When the rectangular pulse MPLSis applied to the base B of the transistor 32, the transistor 32 isplaced in a closed state and held in this closed state for a timeduration TINT which corresponds to a width of the pulse MPLS. Uponexpiration of the time duration TINT, the transistor 32 is returned toan open state. As a result, the primary voltage V1 changes to exhibit acomplicated waveform as indicated in FIG. 5. The waveform of FIG. 5 is awaveform of the primary voltage V1 when the spark plug 10 is normallyinstalled in the engine 48 in the absence of both the discharge gapdefect and the porcelain defect. Waveforms of the primary voltage V1 inthe presence of the discharge gap defect or the porcelain defect will bedescribed below.

As discussed below, the inspection method according to the presentembodiment of the invention is formulated to inspect the spark plug 10on the basis of a variation of the primary voltage V1 after theexpiration of the time duration TINT. Since the absolute value of thesecondary voltage V2 is substantially proportional to that of theprimary voltage V1 after the expiration of the time duration TINT, theinspection may be achieved on the basis of the secondary voltage V2,with substantially the same accuracy. However, the secondary voltage V2is usually as high as 10,000 volts or higher, requiring a voltagemeasuring device to have an accordingly high resistance to voltage, andresulting in increased complexity of the arrangement of the inspectionsystem and accordingly increased cost of manufacture of the system. Inthe light of this fact, the present embodiment is adapted to effect theinspection on the basis of the primary voltage V1.

An electromagnetic energy stored in the primary coil 42 during the timeduration TINT is consumed after the expiration of the time durationTINT, primarily for discharging or sparking at the discharge gap of thespark plug 10. Initially, a so-called “capacitive discharge” takes placeas indicated in FIG. 5. This capacitive discharge causes the secondarycircuit to be formed on the secondary side of the ignition coil 30. Thevoltage required for initiating the capacitive discharge is a voltage(initial breakdown voltage) which is high enough to initiate thedischarge before the formation of the secondary circuit. Upon initiationof the capacitive discharge as indicated by the waveform in FIG. 5, thesecondary voltage V2 instantaneously rises, since the secondary voltageV2 is substantially proportional to the primary voltage V1, as describedabove, owing to the step-up transformer connection of the primary andsecond coils 42, 44 of the ignition coil 30. Therefore, the waveform ofthe secondary voltage V2 can be approximated by observing the waveformof the primary voltage V1 indicated in FIG. 5, except a portion of thewaveform during the time duration TINT.

Following the capacitive discharge, a so-called “inductive discharge”occurs. This inductive discharge is initiated after the secondarycircuit has been formed by the capacitive discharge. Accordingly, theprimary voltage V1 is lower during the inductive discharge than duringthe capacitive discharge. Since the electromagnetic energy graduallydecreases in the process of the inductive discharge, the detectedprimary voltage V1 also gradually decreases during the inductivedischarge. In the last stage of the inductive discharge, the detectedprimary voltage V1 again increases for a relatively short time, as alsoindicated in FIG. 5. In this respect, it is noted that the secondarycircuit which was formed by the capacitive discharge and has been keptduring the inductive discharge tends to be unstable with a decrease inthe inductive discharge energy. Accordingly, an increase in thesecondary voltage V2 is required to hold the capacitive discharge,whereby the above-indicate increase in the primary voltage V1 takesplace. Upon termination of the inductive discharge, the primary andsecondary voltages V1, V2 are both lowered to the line voltage Vss.

Upon reception of the first control signal CTRLC1 from the controldevice 24, the monitoring device 38 determines whether the spark plug 10suffers from a discharge gap defect or not. This determination iseffected based on the detected primary voltage V1. The monitoring device38 applies to the control device 24 a signal RSLT indicative of a resultof the determination, as described below.

The second control signals CTRLC2 and CTRLJ2 are applied from thecontrol device 24 to the ignition control device 36 and the monitoringdevice 38, respectively, for inspecting the spark plug 10 for theporcelain defect. When these second control signals CTRLC2, CTRLJ2 aregenerated from the control device 24, the motor 40 is operated at apredetermined constant speed. As shown in FIG. 6, the engine 48 to beinspected and the motor 40 are mounted on a base 52, and the crankshaftof the engine 48 is connected to the drive shaft of the motor 40 througha coupling 54 and a drive shaft 56. The drive shaft 56 is rotatablysupported by two bearings 58 such that the drive shaft 56 is not axiallymovable. It will be understood that the motor 40, base 52, coupling 54and drive shaft 56 constitute a major portion of an external drivedevice for operating the engine 48. Thus, the inspection of the sparkplug 10 for the porcelain defect is achieved by so-called “motoring” ofthe engine 48 by operation of the motor 40, which should bedistinguished from the so-called “firing” of the engine 48 by combustionof the air-fuel mixture.

Upon reception of the second control signal CTRLC2 from the controldevice 24, the ignition control device 36 generates the rectangularpulse MPLS each time the device 36 receives a TOP DEAD CENTER signal TDCfrom the engine 48. The device 36 generates the rectangular pulse MPLSat a timing which is determined by the moment of generation of thesignal TDC, the speed of rotation of the crankshaft of the engine 48 bythe motor 40, and the position of the cylinder equipped with the sparkplug 10 to be inspected. This timing is determined so that the pressurein the cylinder in question is increased to a level substantially equalto the maximum value. The TOP DEAD CENTER signal TDC is produced from acrank angle sensor for detecting the angular position of the crankshaftof the engine 48. In the present embodiment, the ignition control device36 receives the signal TDC each time the crankshaft is rotated through360° or by one full turn. The above-indicated timing is determined sothat the rectangular pulse MPLS is generated when the piston of thecylinder equipped with the spark plug 10 to be inspected is located atthe top dead center or in the vicinity thereof while the intake andexhaust valves are both closed.

The generation of the pulse MPLS for inspection of the spark plug 10 forthe porcelain defect when the cylinder piston is located at or near thetop dead center is intended to increase the fluid pressure in thevicinity of the discharge gap of the spark plug 10. Generally, thepotential (hereinafter referred to as “sparking voltage Vs”) between twoelectrodes in a gas, which is required to initiate sparking (generate orcreate a spark) across the electrodes, is influenced by the pressure ofthe gas. This phenomenon is known as “Paschen's law” (Paschen's rule),which is indicated by a graph of FIG. 7 in the case where the gas is theair. In the graph of FIG. 7, a product p·d is taken along the horizontalaxis while the sparking voltage Vs is taken along the vertical axis. Thevalue “p” is the pressure of the air, while the value p·d is a distancebetween the center electrode 12 and the grounding electrode 14, namely,the size of the discharge gap. The curve in the graph indicates that thesparking voltage Vs is substantially proportional to the product p·dwhen the product p·d is larger than a value corresponding to a minimumvalue Pmin (so-called “Paschen's minimum”) of the sparking voltage Vs.

In the case of the air, it is known that the Paschen's minimum Pmin isestablished when the discharge gap “d” is approximately 8 pm and thesparking voltage Vs is approximately 325V, while the air pressure “p” isequal to the atmospheric pressure. Generally, the discharge gap “d” ofthe spark plug ranges from 0.6 mm to 1.1 mm, and the air pressure “p” atthe time and place of inspection of the spark plug is equal to theatmospheric pressure or higher. Therefore, the product p·d issufficiently larger than the value corresponding to the Paschen'sminimum Pmin. Thus, the Paschen's law may be approximated by thefollowing linear expression (1):

Vs=≈K·p·d+d  (1)

wherein, K and C are constants, which are positive values in the case ofthe air.

Since the discharge gap size “d” of the specific spark plug 10 to beinspected is constant, the sparking voltage Vs is theoreticallyproportional to the air pressure “p”.

According to the above expression (1), the sparking cannot take placeunless the sparking voltage Vs is suitably increased with an increase inthe air pressure “p”. The sparking at the discharge gap of the sparkplug 10 can be restrained by increasing the air pressure “p” at thedischarge gap. Based on this fact, sparking due to the pin holes orcracks in the porcelain 16 can be initiated before the sparking occursacross the discharge gap. That is, the porcelain defect can be detectedby increasing the air pressure “p”, by the so-called “motoring”indicated above, that is, by bringing the piston of the cylinder inquestion to or near the top dead center by operating the engine 48 bythe motor 40, or by any other suitable method such as introduction ofcompressed air into the cylinder. However, the “motoring” is desirablewhere the external drive device including the motor 40 is used tooperate the engine 48 for inspecting other elements of the engine 48than the spark plug 10. The inspection by the “motoring” of the engine48 using the motor 40 is easier than the inspection by the “firing” ofthe engine 48 by combustion of an air-fuel mixture. Further, the amountof change in the pressure within the engine cylinder is smaller in thecase of the “motoring” than in the case of the “firing”, so that theinspection of the engine 48 by the “motoring” can be generally achievedwith higher accuracy. In this respect, the inspection of the elementsother than the spark plug is usually effected by motoring the engine 48.In this case, the external drive device can be conveniently utilized forinspecting the spark plug 10.

The maximum value of the air pressure “p” is set to be close to a levelat which the sparking takes place at the discharge gap in the absence ofthe porcelain defect. When the spark plug 10 does not have the porcelaindefect, the primary voltage V1 detected during the inspection with the“motoring” of the engine 48 has a waveform similar to that shown in FIG.5 (and FIG. 8 which will be referred to). In this case, the peak of theprimary voltage V1 during the capacitive discharge is higher than in thenormally installed state of the spark plug without the “motoring”, butthe change of the primary voltage V1 during the inductive discharge issubstantially identical with that in the normally installed statewithout the “motoring”.

Upon reception of the second control signal CTRLJ2 from the controldevice 24, the monitoring device 38 determines whether the spark plug 10suffers from a porcelain defect or not. This determination is effectedbased on the detected primary voltage V1. The monitoring device 38applies to the control device 24 a signal indicative of a result of thedetermination, as described below.

Referring to the graphs of FIGS. 8-10, there will be explained themanners of inspecting the spark plug 10 for the discharge gap defect andthe porcelain defect. The waveform of the primary voltage V1 shown inFIG. 8 is a waveform obtained in the normally installed state of thespark plug 10 without the “motoring” of the engine 48 (withoutcontrolling the air pressure “p” as described above). The waveform ofthe primary voltage V1 shown in FIG. 9 is a waveform obtained in thepresence of only the discharge gap defect, during the inspection withoutthe “motoring” of the engine 48, while the waveform of the primaryvoltage V1 shown in FIG. 10 is a waveform obtained in the presence ofonly the porcelain defect, during the inspection with the “motoring” ofthe engine 48. The inspections of the spark plug 10 in the presentembodiment are based on a time parameter value Tn and a voltageparameter value Vn, which are indicated in FIGS. 8-10. The timeparameter value Tn is a time duration (hereinafter referred to as“voltage measuring time”) from a moment at which the primary voltage V1has been raised to a first threshold VTH1 immediately after theexpiration of the time duration TINT, and a moment at which the primaryvoltage V1 has been lowered down to a second threshold VTH2 after it isonce raised above the second threshold VTH2. In the present embodiment,the first threshold value VTH1 is equal to the line voltage Vss, and thesecond threshold VTH2 is slightly higher than the line voltage Vss. Thevoltage parameter value Vn is calculated according to the followingequation (2), on the basis of a surface area A indicated by hatching inFIGS. 8-10 and the time parameter value Tn:

Vn=A/Tn  (2)

That is, the voltage parameter value Vn is an average of the primaryvoltage V1 during the voltage measuring time Tn.

When the spark plug 10 suffers from the discharge gap defect, theprimary voltage V1 during the inductive discharge is lower than when thespark plug 10 is normally installed in the engine without the dischargegap defect, as is apparent from FIG. 9. In the presence of the dischargegap defect, the sparking voltage Vs is lowered due to a relatively smallsize “d” of the discharge gap, as is understood from the aboveexpression (1), and the sparking is likely to take place across thedischarge gap, at a relative low level of the primary voltage V1. In thepresence of the discharge gap, the time parameter value Tn is largerthan in the normally installed state of the spark plug 10, because theduration of the sparking increases with a decrease in the voltage duringthe inductive discharge, provided that the amount of the electromagneticenergy stored in the primary coil 42 in the time duration TINT isconstant. Accordingly, the voltage parameter value Vn which is anaverage of the primary voltage V1 for the voltage measuring time Tn issmaller in the presence of the discharge gap defect than in the normallyinstalled state of the spark plug 10.

When the spark plug 10 suffers from the porcelain defect, the waveformof the primary voltage V1 has a tendency reversed with respect to thatof the waveform in the presence of the discharge gap defect, as shown inFIG. 10. Namely, the primary voltage V1 during the inductive dischargeis higher than in the normally installed state of the spark plug 10, andthe time parameter value Tn is smaller than in the normally installedstate. Accordingly, the voltage parameter value Vn is larger than in thenormally installed state. It is again noted that the waveform of FIG. 10in the presence of the porcelain defect is obtained with the airpressure in the engine cylinder in question being raised by motoring theengine 48, while the waveform of FIG. 9 in the presence of the dischargegap defect is obtained without the “motoring” of the engine 48.

Another parameter value called a shape parameter value Sn is introducedfor the inspection. This shape parameter value Sn is expressed by thefollowing equation (3):

Sn=Vn/Tn  (3)

The shape parameter value Sn is a parameter which generally represents ageometric characteristic of an approximate geometry of the area (surfacearea A) indicated by hatching in FIGS. 8-10. That is, the area isconsidered to be a rectangle whose length and width are represented bythe time parameter value Tn and the voltage parameter value Vn,respectively. The shape parameter value Sn represents a ratio of thewidth (Vn) to the length (Tn). This rectangle whose surface area isrepresented by A is indicated by phantom line (one-dot chain line) inFIGS. 8-10. It will be understood that the shape parameter values Sn ofthe rectangles shown in FIGS. 8-10 have a relationship indicted by thefollowing inequality (4)

Sn in FIG. 9<Sn in FIG. 8<Sn in FIG. 10  (4)

Based on this fact, it is possible to detect the presence of thedischarge gap defect (FIG. 9) when the shape parameter value Sn issmaller than the value in the normally installed state of the spark plug10 (FIG. 8), and detect the presence of the porcelain defect (FIG. 10)when the shape parameter value Sn is larger than the value in thenormally installed state. The present embodiment is adapted to inspectthe spark plug 10 in this way. The inspection may be effected by usingonly the voltage parameter value Vn in place of the shape parametervalue Sn. However, the inspection based on the shape parameter value Snis preferred to the inspection based on only the voltage parameter valueVn, because the shape parameter value Sn has a higher S/N ratio since itis a ratio of the voltage and time parameter values Vn and Tn which haveopposite tendencies of change in the presence of the discharge gapdefect and the porcelain defect.

When the engine 48 is not controlled by the “motoring” by the motor 40as described above, the sparking will take place at the discharge gap,even if there exists the porcelain defect, because the size of thedischarge gap is smaller than the distance between the center electrode12 and the shell 18 through the crack of the porcelain 16. Thus, thesparking will take place at the discharge gap irrespective of thepresence or absence of the porcelain defect, when the “motoring” is noteffected. Accordingly, the inspection of the spark plug 10 for thedischarge gap defect can be achieved with high accuracy irrespective ofwhether there exists the porcelain gap or not.

When the engine 48 is controlled by the “motoring”, on the other hand,that is, when the air pressure in the vicinity of the discharge gap israised at the time the rectangular pulse MPLS is generated to energizethe spark plug 10, the sparking will take place between the centerelectrode 12 and the shell 18 before the sparking at the discharge gap,if the porcelain has cracks while there does not exist the discharge gapdefect. If the size of the discharge gap is smaller than the nominalvalue due to the discharge gap defect, the sparking will not necessarilytake place between the center electrode 12 and the shell 18 before thesparking at the discharge gap. In view of this analysis, the presentembodiment is adapted to effect the inspection for the porcelain defect,for only the spark plug 10 which has been found to have the normaldischarge gap, in the inspection for the discharge gap defect. The sparkplug 10 having the discharge gap defect should be replaced with a newone, irrespective of whether this plug 10 also suffers from theporcelain defect or not. Therefore, the spark plug 10 having thedischarge gap defect need not be inspected for the porcelain defect. Thespark plug 10 installed in place of the defective spark plug is alsosubjected to the inspections for the discharge gap defect and theporcelain defect in the manners described above.

The monitoring device 38 is adapted to execute an ignition monitoringroutine illustrated in the flow chart of FIG. 11. The monitoring device38 includes a processing unit, a ROM (read-only memory) and a RAM(random-access memory). The ROM stores an ignition monitoring programaccording to which the processing unit executes the ignition monitoringroutine of FIG. 11 while utilizing temporary data storage function ofthe RAM. However, the monitoring device 38 may use a magnetic disk ortape or other suitable recording medium which stores various programsincluding the ignition monitoring program and which is accessible by asuitable retrieving device for storing the retrieved program in the RAMor similar memory when the spark plug 10 is inspected.

The monitoring device 38 executes the ignition monitoring routine ofFIG. 11 each time the rectangular pulse MPLS described above is receivedfrom the ignition control device 36. The monitoring device 38 includes asuitable waveform obtaining device, which stores the obtained waveformsof the primary voltage V1 in the RAM. When the waveforms are stored inthe RAM, the waveforms may be subjected to a smoothing operationdescribed below. The monitoring device 38 receives the above-indicatedcontrol signals CTRLJ from the control device 24, immediately before therectangular pulse MPLS is received from the ignition control device 36.As described below, the ignition monitoring routine executed when thecontrol signal CTRLJ1 is received for the inspection for the dischargegap defect is more or less different from the ignition monitoringroutine executed when the control signal CTRLJ2 is received for theinspection for the porcelain defect. Therefore, the control signalsCTRLJ should be received before the reception of the rectangular pulseMPLS which triggers the monitoring device 38 to initiate the routine.

The ignition monitoring routine is initiated with step S100 to initiatea counter variable “i”. Step S100 is followed by step S102 to calculatethe shape parameter value Sn on the basis of the detected waveform ofthe primary voltage V1 stored in the RAM. Then, the control flow goes tostep S104 to effect a determination on the basis of the calculated shapeparameter value Sn. Described more specifically, when the monitoringdevice 38 has received the control signal CTRLJ1 to inspect the sparkplug 10 for the discharge gap defect, step S104 is implemented todetermine whether the following inequality is satisfied or not:

J1←(Sn0−δ)≦Sn  (5)

A decision obtained by the determination is stored as a variable J1. Inthis inspection for the discharge gap defect, the engine 48 is notcontrolled by the “motoring” by the motor 40. The value Sn0 is anaverage of the shape parameter values Sn of a large number of sparkplugs 10 which are normally installed in the engine 48. The value “67”is a predetermined positive value, which may be three times a standarddeviation a which is calculated when the average value Sn0 iscalculated. The value (Sn0−δ) in the inequality (5) may be replaced by aminimum value of the shape parameter values Sn of the large number ofspark plugs 10 in the normally installed state. If an affirmativedecision (YES) is obtained in step S104, it indicates that the dischargegap of the spark plug 10 under examination is normal. Although thedetermination according to the inequality (5) is a determination as towhether the discharge gap is smaller than a predetermined lower limit.However, the inequality (5) may be replaced by the following inequality(6) which permits a determination as to whether the size of thedischarge gap is held in a predetermined range defined by predeterminedlower and upper limits. In this case, the discharge gap whose size islarger than the upper limit (of the discharge gap in the normallyinstalled state of the spark plug) is also determined to be defective.

J1′←(Sn0−δ)≦Sn≦(Sn0+δ)  (6)

In this case, a decision obtained by the determination is stored as avariable J1′.

When the monitoring device 38 has received the control signal CTRLJ2 forthe inspection for the porcelain defect, on the other hand, step S104 isimplemented to determine whether the following inequality is satisfied:

J1←(Sn0−δ)≦Sn  (7)

A decision obtained by this determination is stored as a variable J2. Inthis inspection for the discharge gap defect, the engine 48 iscontrolled by the “motoring” by the motor 40.

If a negative decision (NO) is obtained in step S104, that is, if theabove-indicated inequality (5) or (6) or inequality (7) is notsatisfied, it indicates that there exists the discharge gap defect orthe porcelain defect. In this case, the control flow goes to step S106in which a variable RSLT is set to “NG” indicating that the spark plug10 is defective in its the discharge gap size or porcelain 16. Then,step S108 is implemented to apply a signal RSLT indicative of thevariable RSLT to the control device 24, as indicated in FIG. 4. If anaffirmative decision (YES) is obtained in step S104, that is, if theabove-indicated inequality (5) or (6) or inequality (7) is satisfied, itindicates that the spark plug 10 is normal in its discharge gap size andthe porcelain 16. In this case, the control flow goes to step S110 todetermine whether the counter variable “i” is equal to a preset numberN1 or N2. The preset number N1 is used for the inspection for thedischarge gap defect, while the present number N2 is used for theinspection for the porcelain defect. If an affirmative decision (YES) isobtained in step S110, the control flow goes to step S112 to set thevariable RSLT to “OK” indicating that the spark plug 10 is normal. StepS112 is followed by the step S108 described above, and one cycle ofexecution of the routine is terminated. If a negative decision (NO) isobtained in step Sllo, the control flow goes to step SS4 to incrementthe counter variable “i” and then to step S102. Steps S102, S104, S110and S114 are repeatedly implemented until the counter variable “i” hasbecome equal to the preset number N1 or N2.

The control device 24 is adapted to eventually determine that the sparkplug 10 under examination is normal, only if the affirmative decision(YES) is consecutively obtained in step S104 the number of times equalto the present number N1. In this respect, it is noted that theaffirmative decision (YES) is not necessarily obtained when thedischarge gap is abnormally small, because the sparking does not alwaystake place at the smallest gap, namely, at the defective discharge gap.However, if the inspection is repeated a relatively large number oftimes, there is a high possibility that the affirmative decision (YES)is obtained at least once. This number of time is determined byexperiments as the preset number N1. Thus, if the affirmative decisionis consecutively obtained in step S104 by the present number Ni oftimes, it means that the determination that the spark plug 10 is normalin its discharge gap is highly reliable. Accordingly, the presentarrangement is effective to avoid erroneous determination that thedischarge gap is normal, when the discharge gap is in fact defective.For the same purpose, the inspection for the porcelain defect isrepeated by the present number N2 of time, which is also determined byexperiments. In the manner described above, the control device 24determines the state of the spark plug 10 on the basis of the signalsRSLT received from the monitoring device 38.

Referring to the circuit diagram of FIG. 12, there is shown anarrangement of the monitoring device 38 according another embodiment ofthe present invention. The circuity shown in FIG. 12 corresponds to thefunctions of the processing unit, ROM and RAM which are adapted toexecute the ignition monitoring routine of FIG. 11. The presentmonitoring system 38 of FIG. 12 includes an ENABLE signal generator 70which generates an ENABLE signal EN which is held “ON” during thevoltage measuring time described above, and “OFF” during the other time.The ENABLE signal generator 70 receives a smoothed primary voltage V1Swhich is the primary voltage V1 whose high-frequency component has beenremoved by a low-pass filter 72. The generator 70 receives also firstand second threshold voltage values VTH1, VTH2 from a constant voltagesource 74. The generator 70 is adapted to generate the ENABLE signal EN,by comparing the smoothed primary voltage V1S with the first and secondthreshold voltage values VTH1, VTH2.

The ENABLE signal EN is turned “ON” when the smoothed primary voltageV1S has been raised to the first threshold value VTH1 for the first timeafter the reception of the rectangular pulse MPLS, and is turned “OFF”when the smoothed primary voltage V1S has been lowered down to thesecond threshold VTH2 after it is once raised above the second thresholdvalue VTH2. The thus generated ENABLE signal EN is applied to anintegrator 80 and a counter 84. The integrator 80 integrates thesmoothed primary voltage V1S while the ENABLE signal EN is held “ON”,and the counter 84 counts the number of clock pulses received from aclock 86 while the ENABLE signal EN is held “ON”. The integrator 80 andcounter 84 are reset to “zero” upon reception of the control signalCTRLJ from the control device 24. The output of the integrator 80corresponds to the surface area A indicated by hatching in FIGS. 8-10,and the output of the counter 84 corresponds to the time parameter valueTn. Accordingly, these outputs are expressed by the surface area A andtime parameter value Tn.

The output of the integrator 80 in the form of the surface area A andthe output of the counter 84 in the form of the time parameter value Tnare applied to a divider 90, which calculates a voltage parameter valueVn (=A/Tn) on the basis of the values A and Tn. The output of thedivider 90 in the form of the voltage parameter value Vn and the outputof the counter 84 in the form of the time parameter value Tn are appliedto a divider 92, which calculates the shape parameter value Sn (=Vn/Tn)on the basis of the values Vn and Tn. The shape parameter value Sn isapplied to a comparator 96, which compares the shape parameter value Snand an output voltage VREF of a constant voltage source 98. Thecomparator 97 generates an output G indicative of a result of thecomparison. The voltage VREF is a predetermined voltage valuecorresponding to an average of the shape parameter values Sn of thespark plugs 10 in the normally installed state. The average of the shapeparameter values Sn is obtained when the engine 48 is at rest withoutthe “motoring” control (with the cylinder bores held at the atmosphericpressure), and also when the engine 48 is controlled by the “motoring”by the motor 40 so that the pressures in the cylinder bores are raisedas described above. An appropriate one of these two average values ofthe shape parameter values Sn is selected by the constant voltage source98 depending upon the received control signal CTRLJ (CTRLJ1 or CTRLJ2),to determine the voltage VREF corresponding to the selected average.When the average values of the shape parameter values Sn are obtained,the standard deviation values a of the values Sn are also obtained. Thecomparator 96 selects one of these standard deviation values a dependingupon the received control signal CTRLJ (depending upon whether theengine is “motored” or not), and effects the comparison of the shapeparameter value Sn and the voltage VREF, while taking into account avalue which is three time the selected standard deviation σ.

Based on the shape parameter Sn, voltage VREF and control signal CTRLJ,the comparator 96 generates the output G corresponding to the variableJ1 (J1′) or J2 described by reference to the flow chart of FIG. 11.Where the control signal CTRLJ is the signal CTRLJ1, a determination asto whether the following inequality (8) or (9) is satisfied is effected:

G1←(VREF−3·σ)≦Sn  (8)

G1′←(VREF−3·σ)≦Sn≦(VREF+3·σ)  (9)

A decision obtained by this determination is stored as the output G inthe form of a variable G1 or G1′. The inequalities (8) and (9)correspond to the above-indicated inequalities (5) and (6),respectively. Where the control signal CTRLJ is the signal CTRLJ2, adetermination as to whether the following inequality (10) is satisfiedis effected:

G2←(VREF+3·σ)  (10)

A decision obtained by this determination is stored as the output G inthe form of a variable G2. The above inequality (10) corresponds to theabove-indicated inequality (7). If an affirmative decision (YES) isobtained in the determination, it means that the spark plug 10 underexamination is normally installed in the engine 48.

The monitoring device 38 is capable of generating the signal RSLTindicative of the ENABLE signal EN, time parameter value Tn, voltageparameter value Vn and output G, as indicated in FIG. 12. The controldevice 24 determines whether the spark plug 10 under examination isnormal or defective, on the basis of the output G. The control device 24receives the output G for one inspection of the spark plug 10,immediately after the ENABLE signal EN has been changed from the “ON”state to the “OFF” state. For accurate inspection of the spark plug 10,it is required to repeat the inspection the predetermined numbers oftimes N1, N2 for checking the spark plug for the discharge gap defectand the porcelain defect, so that the control device 24 determines thatthe spark plug 10 is normal, only if the affirmative decision (YES) isobtained the predetermined numbers of times N1, N2 consecutively, as inthe embodiment of FIG. 11.

A further embodiment of the invention will be described by reference toFIGS. 13-15. In this embodiment, the spark plug is inspected on thebasis of a quantity relating to only the level of the primary voltageV1.

The graph o FIG. 13 is a graph wherein the waveforms of the primaryvoltage V1 shown in FIGS. 8, 9 and 10 are superimposed on each other.The waveforms shown in FIG. 13, however, are the waveforms after theyhave been smoothed by a suitable hardware device such as the low-passfilter 72 shown in FIG. 12, or by a software processing equivalent tothe processing by such a hardware device. Based on the thus smoothedwaveforms of the primary voltage V1 (referred to as “smoothed primaryvoltage V1S” as in the embodiment of FIG. 12), the processing for theinspection can be made easier and more accurate. In the presentembodiment of FIG. 13, the inspection of the spark plug 10 is effectedon the basis of the smoothed primary voltage V1S a predetermined timeafter the smoothed primary voltage V1S has been raised to the firstthreshold VTH1. The predetermined time is represented by a variable“offset”. Since the present embodiment does not use the time parametervalue Tn, the second threshold VTH2 used in the preceding embodiment isnot used for the inspection.

The variable “offset” is set to a value corresponding to a time periodduring which the discharge occurring on the spark plug is substantiallycapacitive. This time period can be empirically determined. As isapparent from the graph of FIG. 13, the rate of decrease of the smoothedprimary voltage V1S during the period of the substantially inductivedischarge is comparatively low. Further, the rate of decrease of thesmoothed primary voltage V1S generally decreases as the time passesduring the period of the inductive discharge. Immediately before the endof the period of the substantially inductive discharge, however, therate of decrease of the value V1S increases. In the present embodiment,the inspection of the spark plug 10 is effected based on an average(meanV1S) of the level of the smooth primary voltage V1S while the rateof decrease of the smoothed primary voltage V1S continues to generallydecrease. As is apparent from the graph of FIG. 13, the average meanV1Sis the smallest in the presence of the discharge gap defect, and thelargest in the presence of the porcelain defect, while it isintermediate in the normally installed state of the spark plug.

In the present embodiment, an ignition monitoring routine illustrated inthe flow chart of FIG. 14 is executed in place of the routine of FIG.11. The routine of FIG. 14 is formulated to inspect the spark plug 10 onthe basis of the average meanV1S of the smoothed primary voltage V1S.The ignition monitoring routine 14 is executed by the monitoring device38 similar to that adapted to execute the routine of FIG. 11.

The ignition monitoring routine of FIG. 14 is initiated with step S200to reset the variable “i” to zero. Then, step S202 is implemented toexecute a sub-routine for calculating the average meanV1S. Thissub-routine will be explained by reference to the flow chart of FIG. 15.Then, the control flow goes to step S204 to determine whether thefollowing inequality (11) is satisfied or not:

J3←(meanV1S0−δ)≦meanV1S≦(meanV1S)+δ)  (11)

A decision obtained in this determination is stored as a variable J3.The value “meanV1S0” is an average of the average values meanV1S of alarge number of spark plugs normally installed in the engine 48. Thevariable “σ” is a value three times the standard deviation σ calculatedwhen the average meansV1S0 is calculated. In the present embodiment, theabove-indicated inequality (11) is used for inspecting the spark plugfor both the discharge gap defect and the porcelain defect. If anegative decision (NO) is obtained in step S204, it indicates that thereexists the discharge gap defect or the porcelain defect. In theinspection for the discharge gap defect, the engine 48 is held at restwithout the “motoring” by the motor 40. In the inspection for theporcelain defect, the engine 48 is controlled by the “motoring” by themotor 40 to raise the cylinder pressure when the rectangular pulse MPLSis generated.

If the negative decision (NO) is obtained in step S204, the control flowgoes to steps S206 and S208 similar to steps S106 and S108 of FIG. 11,and one cycle of execution of the routine of FIG. 14 is terminated. Ifan affirmative decision (YES) is obtained in step S204, the control flowgoes to step S210 to determine whether the variable “i” is equal to thepredetermined number N1 or N2. If an affirmative decision (YES) isobtained in step S210, the control flow goes to step S212 and S208similar to step S112 and S208 of FIG. 11, and one cycle of execution ofthe routine is terminated. If a negative decision (NO) is obtained instep S210, the control flow goes to step S214 to increment the variable“i”. Then, steps S202, S204, S210, S214 are repeatedly implemented untilthe affirmative decision is obtained in step S210.

The sub-routine for calculating the average meansV1S is executed in stepS202 according to the flow chart illustrated in FIG. 15. Thissub-routine is initiated with step S300 to set a variable “j” to thevariable “offset”, and reset a variable “ΣV1S” and a variable “n” tozero. Then, step S302 is implemented to add the smoothed primary voltageV1S[j] to the variable “ΣV1S”. Then, the control flow goes to step S304to increment the variable “n”. The variable “j” is equal to “0” at themoment when the smoothed primary voltage V1S has been raised to thethreshold value VTH1 indicated in FIG. 13. The values of the smoothedprimary voltage V1S[j] obtained after the voltage V1S has been raised tothe threshold value VTH1 until the voltage V1S has been stabilized atthe line voltage Vss are successively obtained and stored in the RAM ofthe monitoring device 38.

Then, step S307 is implemented to calculate a variable ΔV1S1 and avariable ΔV1S2 according to the following equations (12) and (13):

ΔV1S=V1S[j+step]−V1S[j]  (12)

ΔV2S=V1S[j+2·step]−V1S[j+step]  (13)

The variable ΔV1S corresponds to the rate of decrease of the smoothedprimary voltage V1S at a point of time indicated by the variable “j”. Onthe other hand, the variable ΔV2S corresponds to the rate of change ofthe smoothed primary voltage V1S at a point of time which is later thanthe point of time indicated by the variable “j” by a time indicated by avariable “step”.

The control flow then goes to step S308 to determine whether thevariable ΔV1S1 is equal to or smaller than a value (ΔV1S2+α). The value“α” is a predetermined variable. If an affirmative decision (YES) isobtained in step S308, the control flow goes to step 310 to add thevariable “step” to the variable “j”, and goes back to step S302. StepsS302, S304, S306, S308 and S310 are repeatedly implemented until anegative decision (NO) is obtained in step S308. The affirmativedecision (YES) obtained in step S308 indicates that the rate of decreaseof the smoothed primary voltage V1S is smaller than a predeterminedvalue. If the negative decision (NO) is obtained in step S308, itindicates that the smoothed primary voltage V1S has begun to increase,namely, it indicates that the period of the inductive discharge is aboutto be terminated. In this case, the control flow goes to step S312 tocalculate the average meanV1S according to the following equation (14):

meanV1S=ΣV1S/n  (14)

Thus, the method of inspecting the spark plug according to the presentembodiment does not require the use of a quantity corresponding to thetime parameter Tn used in the preceding embodiments. It is noted thatthe variable “α” used in step S308 is a predetermined value not smallerthan zero. While the accuracy of detection of the termination of theinductive discharge period deteriorates with an increase of thisvariable “α”, the use of the variable “α” is effective to prevent anerroneous error due to a noise introduced in the detected primaryvoltage V1S.

It will be understood from the foregoing descriptions, the illustratedembodiments are adapted to detect various voltage-related quantitiessuch as the primary voltage V1, smoothed primary voltage V1S, surfacearea A, voltage parameter value Vn. shape parameter value Sn, secondaryvoltage V2, variables ΔV1S1, 66 V1S2 and variable ΣV1S.

While the several preferred embodiments of the invention have beendescribed for illustrated purpose only, it is to be understood that thepresent invention may be embodied with various changes and improvements,without departing from the scope of the invention defined in thefollowing claims.

INDUSTRIAL APPLICABILITY

As described above, the method and apparatus according to the presentinvention permit accurate inspection of a spark plug for the presence ofdefects while the spark plug is installed in an engine.

What is claimed is:
 1. A method of inspecting a spark plug while thespark plug is installed in an engine, said method comprising the stepsof: applying to said spark plug a voltage high enough to cause saidspark plug to generate a spark, while said engine is held at rest andwithout supplying said engine with a fuel; obtaining at least onevoltage-related quantity relating to said voltage applied to said sparkplug; and determining a state of said spark plug on the basis of said atleast one voltage-related quantity.
 2. A method according to claim 1,wherein said spark plug is energized by application of said voltagethereto while a pressure in a cylinder of said engine containing thespark plug is held at an atmospheric level.
 3. A method according toclaim 1, wherein said spark plug is energized by application of saidvoltage thereto while a pressure in a cylinder of said engine containingthe spark plug is held at a level higher than an atmospheric pressure.4. A method according to claim 3, wherein the pressure in said cylinderis substantially equal to a pressure in said cylinder when an air-fuelmixture in said cylinder is ignited by combustion of a fuel duringnormal operation of said engine.
 5. A method according to claim 4,wherein compressed air is introduced into said cylinder.
 6. A methodaccording to claim 1, wherein said spark plug is energized with asecondary voltage generated by a voltage applying device on the basis ofa primary voltage lower than said secondary voltage, and said at leastone voltage-related quantity includes at least one quantity relating toat least one of said primary and secondary voltages.
 7. A methodaccording to claim 6, wherein said at least one quantity includes a timeduration (Tn) of a discharge which occurs on the spark plug energized toproduce said spark.
 8. A method according to claim 7, wherein said atleast one quantity includes an average (Vn) of said at least one of saidprimary and secondary voltages.
 9. A method according to claim 8,wherein said at least one quantity includes a ratio (Sn) of said averageto said time duration (Tn) of said discharge.
 10. A method according toclaim 6, wherein said step of obtaining at least one voltage-relatedquantity comprises detecting said primary voltage, and obtaining said atleast one quantity relating to said primary voltage.
 11. A methodaccording to claim 10, wherein said at least one quantity relating tosaid primary voltage comprises an average of said primary voltage.
 12. Amethod according to claim 11, wherein said at least one quantityrelating to said primary voltage comprises a time duration of adischarge which occurs on said spark plug energized to generate saidspark.
 13. A method according to claim 12, wherein said at least onequantity relating to said primary voltage comprises a ratio of saidaverage to said time duration.
 14. A method according to claim 10,wherein said spark plug is energized by an external voltage applyingdevice.
 15. A method according to claim 10, wherein said spark plug isenergized with said secondary voltage while a pressure in a cylinder ofsaid engine in which said spark plug is installed is held at anatmospheric level.
 16. A method according to claim 15, wherein saidspark plug is energized with said secondary voltage while said engine isheld at rest.
 17. A method according to 10, wherein said spark plug isenergized with said secondary voltage while a pressure in a cylinder ofsaid engine in which said spark plug is installed is held at a levelhigher than an atmospheric level.
 18. A method according to claim 17,wherein said spark plug is energized with said secondary voltage whilesaid engine is operated by an external drive device.
 19. A methodaccording to claim 17, wherein said spark plug is energized by saidsecondary voltage while a piston of said engine is located at a top deadcenter thereof.
 20. A method according to claim 17, wherein said sparkplug is energized with said secondary voltage while compressed air isintroduced into said cylinder while said engine is held at rest.
 21. Amethod of inspecting a spark plug while the spark plug is installed inan engine, said method comprising the steps of: energizing said sparkplug with a secondary voltage generated on the basis of a primaryvoltage lower than said secondary voltage, without supplying said enginewith fuel, such that said secondary voltage is high enough to cause saidspark plug to generate a spark; detecting said primary voltage;obtaining at least one quantity relating to said primary voltage; anddetermining a state of said spark plug on the basis of said at least onequantity.